Nicotinic receptors are targets for a great number of exogenous and endogenous compounds that allosterically modulate their function. See, Arias, H. R., Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor, Biochimica et Biophysica Acta—Reviews on Biomembranes 1376: 173-220 (1998) and Arias, H. R., Bhumireddy, P., Anesthetics as chemical tools to study the structure and function of nicotinic acetylcholine receptors, Current Protein & Peptide Science 6: 451-472 (2005). The function of nicotinic receptors can be decreased or blocked by structurally different compounds called non-competitive modulators, including non-competitive antagonists (reviewed by Arias, H. R., Bhumireddy, P., Bouzat, C., Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors. The International Journal of Biochemistry & Cell Biology 38: 1254-1276 (2006)).
Non-competitive modulators comprise a wide range of structurally different compounds that inhibit receptor function by acting at a site or sites different from the orthosteric binding site. Receptor modulation has proved to be highly complex. The mechanisms of action and binding affinities of non-competitive modulators differ among nicotinic receptor subtypes (Arias et al., 2006). Non-competitive modulators may act by at least two different mechanisms: an allosteric and/or a steric mechanism.
An allosteric antagonist mechanism involves the binding of a non-competitive antagonist to the receptor and stabilization of a non-conducting conformational state, namely, a resting or desensitized state, and/or an increase in the receptor desensitization rate.
In contrast, a straightforward representation of a steric mechanism is that an antagonist molecule physically blocks the ion channel. Such antagonists may be termed non-competitive channel modulators (NCMs). Some inhibit the receptors by binding within the pore when the receptor is in the open state, thereby physically blocking ion permeation. While some act only as pure open-channel blockers, others block both open and closed channels. Such antagonists inhibit ion flux through a mechanism that does not involve binding at the orthosteric sites.
Barbiturates, dissociative anesthetics, antidepressants, and certain steroids have been shown to inhibit nicotinic receptors by allosteric mechanisms, including open and closed channel blockade. Studies of barbiturates support a model whereby binding occurs to both open and closed states of the receptors, resulting in blockade of the flow of ions. See, Dilger, J. P., Boguslavsky, R., Barann, M., Katz, T., Vidal, A. M., Mechanisms of barbiturate inhibition of acetylcholine receptor channels, Journal General Physiology 109: 401-414 (1997). Although the inhibitory action of local anesthetics on nerve conduction is primarily mediated by blocking voltage-gated sodium channels, nicotinic receptors are also targets of local anesthetics. See, Arias, H. R., Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors, Neuroscience and Biobehavioral Reviews 23:817-843 (1999) and Arias, H. R. & Blanton, M. P., Molecular and physicochemical aspects of local anesthetics acting on nicotinic acetylcholine receptor-containing membranes, Mini Reviews in Medicinal Chemistry 2: 385-410 (2002).
For example, tetracaine binds to the receptor channels preferentially in the resting state. Dissociative anesthetics inhibit several neuronal-type nicotinic receptors in clinical concentration ranges, with examples such as phencyclidine (PCP) (Connolly, J., Boulter, J., & Heinemann, S. F., Alpha 4-beta 2 and other nicotinic acetylcholine receptor subtypes as targets of psychoactive and addictive drugs, British Journal of Pharmacology 105: 657-666 (1992)), ketamine (Flood, P. & Krasowski M. D., Intravenous anesthetics differentially modulate ligand-gated ion channels, Anesthesiology 92: 1418-1425 (2000); and Ho, K. K. & Flood, P., Single amino acid residue in the extracellular portion of transmembrane segment 2 in the nicotinic α7 acetylcholine receptor modulates sensitivity to ketamine, Anesthesiology 100: 657-662 (2004)), and dizocilpine (Krasowski, M. D., & Harrison, N. L., General anaesthetic actions on ligand-gated ion channels, Cellular and Molecular Life Sciences 55: 1278-1303 (1999)). Studies indicate that the dissociative anesthetics bind to a single or overlapping sites in the resting ion channel, and suggest that the ketamine/PCP locus partially overlaps the tetracaine binding site in the receptor channel. Dizocilpine, also known as MK-801, is a dissociative anesthetic and anticonvulsant which also acts as a non-competitive antagonist at different nicotinic receptors. Dizocilpine is reported to be an open-channel blocker of α4β2 neuronal nicotinic receptors. See, Buisson, B., & Bertrand, D., Open-channel blockers at the human α4β2 neuronal nicotinic acetylcholine receptor, Molecular Pharmacology 53: 555-563 (1998).
In addition to their well-known actions on monoamine and serotonin reuptake systems, antidepressants have also been shown to modulate nicotinic receptors. Early studies showed that tricyclic antidepressants act as non-competitive antagonists. See, Gumilar, F., Arias, H. R., Spitzmaul, G., Bouzat, C., Molecular mechanisms of inhibition of nicotinic acetylcholine receptors by tricyclic antidepressants. Neuropharmacology 45: 964-76 (2003). Garćia-Colunga et al., report that fluoxetine, a selective serotonin reuptake inhibitor (SSRI), inhibits membrane currents elicited by activation of muscle or neuronal nicotinic receptors in a non-competitive manner; either by increasing the rate of desensitization and/or by inducing channel blockade. See, Garćia-Colunga, J., Awad; J. N., & Miledi, R., Blockage of muscle and neuronal nicotinic acetylcholine receptors by fluoxetine (Prozac), Proceedings of the National Academy of Sciences USA 94: 2041-2044 (1997); and Garćia-Colunga, J., Vazquez-Gomez, E., & Miledi. R., Combined actions of zinc and fluoxetine on nicotinic acetylcholine receptors, The Pharmacogenomics Journal 4: 388-393 (2004). Mecamylamine, previously approved for the treatment of hypertension, is a classical non-competitive nicotinic receptor antagonist, and is also well known to inhibit receptor function by blocking the ion channel. See, Giniatullin, R. A., Sokolova, E. M., Di Angelantonio, S., Skorinkin, A., Talantova, M. V., Nistri, A. Rapid Relief of Block by Mecamylamine of Neuronal Nicotinic Acetylcholine Receptors of Rat Chromaffin Cells in Vitro: An Electrophysiological and Modeling Study. Molecular Pharmacology 58: 778-787 (2000).